The invention relates to data transmission between such amplifiers that use optical fiber as their data transmission channel.
A fiber amplifier is a device with which an optical signal can be amplified without converting the signal into an electrical signal in between. These amplifiers are used with very high transmission rates. A typical value of transmission rate may be 1 GHz, for example. The operation of a prior art fiber amplifier is illustrated in FIG. 1. In the figure the conductors transmitting optical signals are indicated with double lines and the conductors transmitting electrical signals are indicated with single lines. An input signal 1 of the amplifier is conducted via an optical isolator 12 to the second input of a wavelength combiner 14. The function of the optical isolator 12 is to attenuate reflections, for example. An input signal detector 16 after the optical isolator 12 detects the level of the input signal and generates an electrical signal corresponding to this level. A pump laser unit 18 supplies the second input of the wavelength combiner 14. The amplification of a signal operates in the following way. The pump laser 18 produces light whose wavelength is e.g. 980 or 1480 nm, whereas the wavelength of the signal light is 1,550 nm, for example. The photons of the pump laser 18 are conduced to an erbium-doped fiber 10 where they excite erbium atoms of the fiber 20. Some of the erbium atoms return to the ground state via spontaneous emission. When the photons of the signal light is directed to the erbium atom tuned by the photons of the pump laser, the erbium atom emits a photon corresponding to the photon of the signal light. The erbium-doped fiber 20 is followed by a band-pass filter 22 passing through the photons displaced by the photons of the signal light, but prevents the travel of the photons which are generated mostly by their spontaneous emission and whose wavelength is not similar to the wavelength of the signal light. An output signal 2 of the band-pass filter is an amplified optical signal transmitted to a (not shown) receiving station. Fiber amplifiers are discussed e.g. in Optical amplifiers and their Applications, edited by S. Shimada and H. Ishido, .COPYRGT.John Wiley & Sons, 1992.
Signal wavelength and characteristics of the band-pass filter 22 vary e.g. according to temperature or change considerably if some component of the system has to be replaced. In that case, the amplifier should be able to be adjust the pass wavelength of the band-pass filter by external, e.g. electrical control. Prior art arrangements comprise a controller 26 for this purpose. The controller 26 monitors the detector 16 of the input signal and a detector 24 of the output signal and amplifies the control signal of the band-pass filter 22 for maximizing the amplification of the amplifier or the correlation between the output and input signals.
As a fiber amplifier normally only amplifies a signal with a certain factor, without adding information to the signal, signalling associated with supervision and control of the amplifier generally has to be carried out with separate systems. This is a particular problem when using a so-called intermediate amplifier which is far (e.g. over 100 km) from both the transmitter and the receiver.
European Patent Application 415 438 discloses an art where supervisory messages are transmitted by modulating the control signal of a pump laser unit in such a manner that the excess amount of pumping light from the amplifier contains a supervisory signal to be transmitted. This prior art solution has, however, several limitations. First, the prior art solution is only applicable on a wavelength of 1,480 nm as the excess pumping light on a wavelength of 980 nm will be absorbed on a long data link by an information transmitting fiber. At the moment, it seems that the use of the wavelength of 1,480 nm is being abandoned as the wavelength of 980 nm produces better noise characteristics to the fiber amplifier and the total efficiency ratio of the amplifier will be better. Secondly, the prior art solution requires two separate optical systems, one for receiving a payload signal and one for receiving a supervisory signal. The reception of supervisory signals according to the prior art requires a separate WDM component (Wavelength Division Multiplexer). Thirdly, the prior art solution is only useable on such frequencies whose period is shorter than the lifetime of the fluorescence state.